A diabatic state model for donor-hydrogen vibrational frequency shifts in hydrogen bonded complexes

A simple diabatic two-state Hamiltonian is considered which describes hydrogen bonding and proton transfer between a donor (D) and an acceptor (A), including different classes of bonds (weak, low-barrier, and strong). The independent variables are the distance R and the difference in proton affinity ϵ between the donor and acceptor. With only two free parameters the model Hamiltonian gives a quantitative description of experimental data from a wide range of molecular complexes exhibiting correlations between R and ϵ and binding energies, D–H bond lengths, and the frequencies of D–H stretch and bend vibrations. The UV photo-dissociation of strong H-bonded complexes via an excited electronic state with an exalted vibrational frequency is predicted.